Servovalve with oscillation filter

ABSTRACT

The pressure variations in the spool end chambers of a two-stage electrohydraulic servovalve having a second-stage sliding-spool type hydraulic amplifier, resulting from oscillation at a characteristic resonant frequency when the servovalve is operated at relatively high pressures and especially with low viscosity hydraulic fluid, are effectively acoustically filtered out by the provision of a wave tube for each spool end chamber having a length equal to one-quarter the wavelength of such frequency.

Eiite States Paent [191 Elarlr SERVOVALVE WITH OSCILLATION FILTER [1 1 3,857,541 Dec. 31, 1974 3,347,252 /1967 Hanson l37/624.l5X

Prima Examiner-Alan Cohan 75 1 t 1 D l W men or z xs Clark waldaecker Assistant Examiner-Gerald A. Michalsky Attorney, Agent, or Firm-Sommer & Sommer [73] Assignee: Moog Inc., East Aurora, NY.

[22] Filed: June 14, 1973 [57] ABSTRACT [21 Appl. N0.: 369,789 The pressure variations in the spool end chambers ofa two-stage electrohydraulic servowalve having a second-stage sliding-spool type hydraulic amplifier. re-

{52] 251/30 '2 gZ sulting from oscillation at a characteristic resonant frequency when the servovalve is operated at rela- 13/043 F161 47/00 tively high pressures and especially with low viscosity [5 g 3 59616 hydraulic fluid, are effectively acoustically filtered out 13 2 6 251/ 36 by the provision of a wave tube for each spool end chamber having a length equal to onequarter the [56] References cued wavelength of such frequency.

UNITED STATES PATENTS 3095,906 7/l963 Kolm l37/625.62 7 Clams, 9 ing Figures gg 93 I3 E 3 92 95 4 2 9 |4| 4 8 I Q us 5 l3 2 I 48 4o 56 0 e0 38 R J A 1 P PATENTED m 1 I914 $557; 541 saw 3 OF Q (PEAK FLOW) FLOW . ZTFx Q QM Q0 cos x I v (PEAK PREssuRE) PO PRESSURE V zn P PM P0 sln X Y -x OPEN END NODE AT INPUT CLOSED END TO TUBE I SPOOL END 4 AV TUBE CHAMBER w E I L Q BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of electrohydraulic servovalves, and especially those which are prone to oscillate under certain operating conditions.

Most present-day, two-stage servovalves comprise a permanent magnet torque motor having a movable armature member; a first-stage hydraulic amplifier, be it of the nozzle-flapper type, the movable jet pipe type, or the deflector jet type, having a movable control member connected to the armature; and a second-stage, sliding-spool type hydraulic amplifier. Such valves usually incorporate mechanical feedback in the form of a spring member that creates torque or force on the armature proportional to spool position. Also, such servovalves generally incorporate a bushing or sleeve member that ports hydraulic fluid to the second-stage spool.

The above known servovalves are sometimes used in systems where very high valve dynamic response is re quired, such as material test machines, motion simulators, vibration exciters, etc. Typically, the servovalves for these systems have moderately high flow control, such as to gallons per minute (gpm) at 1000 pounds per square inch (psi) (38 to 58 litres per minute at 70 bars) and system supply pressures are often 4000 psi (280 bars) or higher. The use of high control flow with high supply pressure provokes servovalve instability tendencies, such as high amplitude resonance or even selfoscillation.

In addition these systems sometimes:

l use laminar seals (metal-to-metal) in the load cylinder for both piston head and rod end seals, to achieve lower friction than obtainable with conventional O-ring seals; r

(2) use hydrostatic bearings to support the driven load to reduce further load drive friction;

(3) mount the servovalve directly on the load cylinder to minimize oil compliance and thereby increase load resonant frequency;

(4) operate with very high position loop gains from the piston position feedback transducer to the servoamplifrer that drives the servovalve, to improve further load dynamic response; and

(5) may operate with signal input commands that are well beyond the normal bandwidth of the position servo to achieve high load excitation frequencies. Although the amplitude ratio at high frequencies may be much reduced from that at low frequencies it is still possible to get meaningful load response at higher frequencies by increasing the amplitude of the command signal.

. The above practices tend to provoke servovalve instabilities, excite servovalve resonances, and cause the load to respond to such servovalve oscillatory outputs;

For years, many such systems have been plagued by oscillation tendencies of servovalves. Prolonged operation with high frequency commands raises the fluid temperature. Reduced viscosity of the fluid at high temperatures, i.e., 150F. (65C.) and above, further provokes oscillation problems. Very low friction between the cylinder and piston, together with close coupling of the servovalve, cause servovalve oscillations to be transmitted to the load. Such servovalve oscillations may cause severe pressure variations in the load cylin der, i.e. $2,000 psi bars) or more, when operating with 4000 psi (280 bars) supply pressure. Such oscillations may become very objectionable, causing an intense squeal or audible noise. They may also result in fatigue failures of the servovalve, cylinder, or load; 0- rings may extrude; and other damage may occur. When high servovalve resonance occurs, internal valve parts may deteriorate to an extent such that the servovalve will sustain the oscillation. This instability may persist even when the electrical signal is completely disconnected from the servovalve.

The oscillation problem frequently is more pronounced when the servovalve is operating near null, i.e., holding a fixed actuator position. Instability at null is provoked by the rapid variations in spool flow forces that occur as the spool passes through null, and is also provoked by the reversal in drive pressures on the spool end areas. This servovalve null instability problem can often be caused by applying step inputs to the system. Such oscillation may especially be a problem if step input commands are frequently applied to the system.

Resonant frequency characteristics of a servovalve are peculiar to each specific valve design. Thus, for duplicates of a given valve design, the resonance may vary in amplitude from individual valve to valve, but the characteristic resonant frequency is the same. Different servovalve designs may produce different characteristic resonant frequencies. Typical resonances of a servovalve having an armature supported on a flexure tube include, for example: 550 to 750 lI-Iertz (I-Iz) first-stage armature/flapper resonance on net stiffness of the flexure tube and magnetic circuit; 3.0 to 3.5 kiloI-Iertz (KI-I2) first-stage armature/flapper resonance on the mechanical stiffness of the flexure tube; still higher first-stage resonant frequencies as the armature/flapper vibrates in harmonic modes; 3.0 to 3.5 KHz resonance of the bushing and spool'masses on the retained stiffness of a bushing locating pin and net compliance of fluid in the spool end chambers; and beat frequencies as the spool and bushing move in and out of phase. The

overall problem may be aggravated and become very serious if the first and second stage resonant frequencies coincide.

2. Description of the Prior Art There has been a long felt need to solve the problem of servovalve oscillation. While there have been many prior attempts to eliminate or reduce servovalve oscillation tendencies, generally they have not been very successful, the techniques tried including:

(1) Reduce internal loop gain of the servovalve by reducing the size of the hydraulic amplifier orifices or reducing the feedback wire stiffness or increasing the spool end areas. These changes generally are not effective in the frequency range above the normal valve operating bandwidth.

(2) Change the inherent resonant frequencies to separate first and second stage individual resonances. These changes include adding armature mass or increasing the stiffness of the bushing retaining pin or preloading the bushing to eliminate backlash at this pin and to obtain a force preload (i.e. zero bushing motion until the preload force is exceeded) or adding fluid volume in the end cap chambers to increase the compliance (i.e., reduce the fluid spring effect).

(3) Dampen or attenuate the amplitude of resonances by adding damping fluid around the torque motor armature, or adding copper rings or shorted turns to get electrical damping of the armature, or adding orifices in fluid passages between the first and second stages, or changing the hydraulic porting between the first and second stages to increase their separation or otherwise decouple the dynamic interactions.

At best, these previous solutions have been only partially effective, within limits imposed by the need to re tain normal servovalve dynamic response, to not add undue machining costs, to retain reasonable valve envelope, and other practical considerations.

SUMMARY OF THE INVENTION The present invention provides a solution to the oscillatory problem of an electrohydraulic servovalve, especially one operating in the elevated pressure range of 3000 to 4000 psi and with low viscosity hydraulic fluid where heretofore the problem was particularly acute, different from and more effective than the aforementioned prior art attempts to solve the problem.

In accordance with the present invention, acoustic frequency type passive hydraulic resonant filters are provided in association with the spool end chambers. The filter for each spool end chamber is a-wave tube having a characteristic length and having a diameter sufficient to give effective filtering. The length of the wave tube equals one-quarter the wave length of the servovalves characteristic resonant frequency to be attenuated. The addition of the filter does not affectthe normal servovalve response in its lower frequency range. Such a resonance filter in the-form of a wave tube is simply and cheaply added, achieved by providing connected drillings in the valve body without complex machining. The diameter of the drilled composite wave tube is not critical. Its location and length is compatible with the basic valve envelope. Moreover, the filter attenuates the oscillation tendency right at the second-stage and so isolates valve oscillations from the driven load.

Accordingly, the main object of the present invention is to provide an effective solution to the problem of servovalve oscillation, especially when operating at a high pressure level and with low viscosity hydraulic fluid.

Other objects are to provide a solution which is simple, inexpensive to achieve, compatible with the basic valve envelope, does not interfere with normal valve response in its lower frequency range, and does not require utilization of any of the prior art damping techniques which were not fully satisfactory anyway.-

Still other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a substantially central longitudinal sectional view through an electrohydraulic servovalve constructed in accordance with the principles of the present invention and illustrating a preferred embodiment thereof, this view being taken generally on line ll of FIG. 2.

FIG. 2 is a vertical transverse sectional view thereof taken on line 2-2 of FIG. 1.

FIG. 3 is a horizontal sectional view thereof taken on line 33 of FIG. 1, and illustrates the longitudinal extents of the elongated tubular cavities of wave tubes formed by drillings in the valve body components which provide the acoustic filter means constituting the improvement of the present invention.

FIG. 4 is a vertical transverse sectional view of the valve body, taken on line 44 of FIG. 1 and showing in elevation the inside face of the left end cap forming one component of the valve body.

FIG. 5 is another vertical transverse sectional view of the valve body, taken on line 5-5 of FIG. 1, and showing in elevation the inside face of the right end cap forming another component of the valve body.

FIG. 6 is a vertical longitudinal sectional view of the left end cap, taken generally on line 6-6 of FIG. 4.

FIG. 7 is a vertical longitudinal sectional view of the right end cap, taken generally on line 7-7 of FIG. 5.

FIG. 8 is a horizontal sectional view of the valve body and other elements housed therein, taken generally on line 88 of FIG. 2.

FIG. 9 is a composite of separate graphs depicting the flow and pressure characteristics of fluid in a wave tube associated with a spool end chamber, also schematically depicted.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electrohydraulic servovalve illustrated in the drawings is a two-stage servovalve shown as comprising a permanent magnet torque motor 10 including a movable armature member 11, a first-stage hydraulic amplifier 12 preferably of the nozzle-flapper type including a movable control member 13 connected to the armature, and a second-stage, sliding spool type hydraulic amplifier 14 including a valve body 15 housing a bushing assembly 16 in which a valve spool 17 is slidably arranged.

Valve body 15 is shown as comprising three components including an intermediate body block 18, a left end cap 19, and a right end cap 20. Block 18 has a horizontal bore 21 therethrough extending from its vertical left end face 22 to its vertical right end face 23. Left end cap 19 has a flat annular inner end face 24, surrounding an inwardly axially projecting cylindrical tu-' bular neck 25, in turn surrounding an annular recess 26, leaving a central axially extending post 28,'the end face 29 of which is slightly outside the plane of annular face 24 and serves as a stop for the left end of valve spool 17. Left end cap 19 is secured to body block 18 by a pair of screws 30. Neck 25 projects into the left end of body bore 21 and is sealed thereto by O-ring 27.

Similarly, right end cap 20 has a flat annular inner face 31, surrounding an inwardly axially projecting cylindrical tubular neck 32, in turn surrounding an annular recess 33, leaving a central axially extending post 34, the end face 35 of which is slightly outside the plane of annular face 31 and serves as a stop for the right end of valve spool 18. Right end cap 20 is secured to body block 18 by a pair of screws 36. Neck 32 projects into the right end of body bore 21 and is sealed thereto by O-ring 37.

Bushing assembly 16 is arranged in body bore 21. while this assembly may be constructed in any suitable form the same preferably comprises as shown a stack of brazed-together tubular parts including a left end part 38, a left intermediate part 39, a center part 40, a

right intermediate part 41, and a right end part 42 to provide a unitary structure. Center bushing part 40 is shown as provided with a chordal slot 43 into which the inner end 44 of a bushing pin 45 projects, this pin being housed within body block 18 and extending generally radially with respect to the bushing assembly. This pin 45 has a sealed connection with body block 18 provided by O-ring 46 and is retained in position by a surrounding sleeve fitting 48 having a threaded connection with the body block. Pin 45 serves to prevent axial movement of bushing assembly 16 relative to body block 18.

Bushing assembly 16 is shown as formed so that jointly with the surrounding wall of body bore 21, there are provided a left annular pressure chamber 50, a left annular outer control chamber 51, a central annular return chamber 52, a right annular outer control chamber 53, and a right annular pressure chamber 54. Left end radial metering passages 55 connect chamber 50 to the cylindrical bore 56 extending horizontally through the bushing assembly. Left and right center radial metering passages 57 and 58 respectively, arranged at the axial ends of center bushing port 40 connect chamber 52 to bore 56. Right end radial metering passages 59 connect chamber 54 to bore 56. Left radial control passages 60 connect chamber 51 to bore 56, and right radial control passages 61 connect chamber 53 to bore 56.

Intermediate bushing parts 39 and 41 are shown sealed to the wall of body bore 21 by a pair of O-rings 62 on opposite sides of left control chamber 51 and a pair of O-rings 63 on opposite sides of right control chamber 53. Left end bushing part 38 is of smaller external diameter than adjacent bushing part 39 and projects into the bore of left end cap neck and is sealed thereto by O-ring 64. Similarly, right end bushing part 42 is of smaller external diameter than adjacent bushing part 41 and projects into the bore of right end cap neck 32 and is sealed thereto by O-ring 65.

Valve spool 17 is shown as including a left lobe 67, a center lobe 68 and a right lobe 69. These lobes are cylindrical in form and slidably engage the wall of bushing bore 56. Left lobe 67 is connected to center lobe 68 by an integral concentric left stem 70 and this center lobe is connected toright lobe 69 by an integral concentric right stem 71. The opposing end faces of lobes 67 and 68 are radial and spaced apart in axial distance corresponding to that between the adjacent edges of metering passages 55 and 57, and with the periphery of stem 71) and the surrounding portion of bore 56 jointly provide a left annular inner control chamber 72. Likewise, the opposing end faces of lobes 68 and 69 are ra' dial and spaced apart an axial distance corresponding to that between the adjacent edges of metering passages 58 and 59, and with the periphery of stem 71 and the surrounding portion of bore 56 jointly provide a right annular inner control chamber 73.

It will be noted that passages 60 communicate inner and outer left control chambers 72 and 51, respectively, and passages 61 communicate inner and outer right control chambers 73 and 53, respectively. Chamber 51 communicates with a first control port C in the base of body block 20. Chamber 53 communicates with a second control port C also in the base of the body block. These control ports are adapted to be connected with a load such as a piston and cylinder actuator (not shown).

The base of body block 18 is also shown as provided with a pressure port P and a return port R. Port R is shown as communicating with central chamber 52. Port P is shown as communicating with a horizontal hole 74 extending through body block 18 alongside bushing bore 21 at a lower level.

Referring to FIG. 8, the end of hole 74 adjacent end cap 19 is externally sealingly plugged by a sleeve plug member 75 which internally supports a fixed orifice member 7. The other end of hole 74 adjacent end cap 20 is externally sealingly plugged by a sleeve member 78 which internally supports a fixed orifice member 79. Extending between sleeve plug members 75 and 78 is a tubular filter screen 80 having a transverse dimension less that that of hole 74 to provide an annular space 81 around this screen.

Pressurized hydraulic fluid applied to port P fills hole 74 including space 81 and occupies pressure chambers 50 and 54 which communicate with this space as shown in FIG. 8. When valve spool 18 is in a null'or centered position, as shown in FIG. 1, left spool lobe 67 blocks communication between left pressure chamber 50 and left inner control chamber 72; right spool lobe 69 blocks communication between right pressure chamber 54 and right inner control chamber 73; and center spool lobe 68 blocks communication between return chamber 52 with both of these control chambers 72 and 73.

Pressurized fluid can also How through filter screen 80 into the interior thereof and thence outwardly through fixed orifices 76 and 79. Downstream of orifice 76the bore 82 of sleeve plug 75 communicates with the inner open end of a horizontal dead-ended passage 83 provided in end cap 19. This passage 83 has a branch passage 84 which communicates with cap recess 26. An O-ring 85 seals the connection between bore 82 and passage 83.

Similarly at the other end of the servovalve, the portion of bore 86 of sleeve plug 78 downstream of orifice 79 communicates with the inner open end of a horizontal dead-ended passage 87 in end cap 20 having a branch passage 88 heading to cap recess 33. An O-ring 89 seals the connection between bore 86 and passage 87.

Referring to FIG. 1, the left cap recess 26 and the outer left end faces of spool bushing assembly 16 and valve spool 17 define a left spool end chamber 91, and the right cap recess 33 and the outer right end faces of this bushing assembly and valve :spool define a right spool end chamber 92. These spool end chambers 91 and 92 are operatively associated with the firststage hydraulic amplifier 12 in the following manner.

This amplifier 12 is shown as including a nozzle block 93 having a flat bottom surface 94 engaging the flat top surface 95 of valve body block 18. Screws 96 secure these blocks together. Nozzle block 93 has a vertical central opening 98 extending therethrough and projecting into this opening from diametrically opposite positions are the tips of left and right nozzles 99 and 100, respectively. These nozzles are suitably supported horizontally on nozzle block 93 in fixed and spaced relation to each other. The lower end portion of control member 13, acting as a flapper, extends in the space between these nozzles and jointly with their tips provide a pair of differentially variable orifices.

The bore or passage of left nozzle 99 communicates with left spool end chamber 91, while the bore or passage of right nozzle 100 communicates with right spool end chamber 92. For this purpose, left nozzle 99 communicates with a chamber 101 in nozzle block 93 from which a passage 102 extends to surface 94. There, passage 102 communicates with a passage 103 in body block 18 which extends between surface 95 and end face 22. At this latter location, the end of passage 103 communicates with one end ofa passage 104 in left end cap 19 which leads to cap recess 26 and hence left spool end chamber 91. An O-ring 105 seals the interface joint between surfaces 94 and 95 around the connected passages 102 and 103, and an O-ring 106 seals the interface joint between surfaces 22 and 24 around the connected passages 103 and 104.

Similarly, right nozzle 100 communicates with a chamber 108 in nozzle block 93 from which a passage 109 extends to surface 94. There, passage 109 communicates with a passage 110 in body block 18 which extends between surface 95 and end face 23. At this latter location, the end of passage 110 communicates with one end of a passage 111 in right end cap 20 which leads to cap recess 33 and hence right spool end chamber 92. An O-ring 112 seals the interface joint between surfaces 94 and 95 around the connected passages 109 and 110, and an O-ring 113 seals the interface joint between surfaces 23 and 31 around the connected passages 110 and 111.

Opening 98 in nozzle block 93 is shown as communicating with the upper end of a tubular union member 114 the upper end of which is sealingly connected to this nozzle block by O-ring 115 and to body block 18 by O-ring 116. The lower end of member 114 extends into a radial hole in the center part 40 of bushing assembly 16 and is sealingly connected thereto by O-ring 118. This member 114 has a drain orifice 119 (FIG. 2) on its side wall which communicates the sum chamber 120 formed by the connected opening 98 and interior of member 114 with annular return chamber 52. Orifice ll9-reduces the effect of any pressure surges that may occur in the return side of the hydraulic system, including chamber 52, upon sump chamber 120 and the fluid flow passages or bores of nozzles 99 and 100 discharging into such sump chamber.

' Torque motor is a polarized electrical force motor of well-known construction. As illustrated, it includes upper and lower pole pieces 121 and 122, respectively, between which permanent magnets 123 extend and house a pair of coils 124 which surround opposite arm portions of armature 11. The tips of this armature extend intoair gaps formed between the opposing ends of flange portions of the pole pieces 121 and 122. The assembly of pole pieces, permanent magnets and coils is mounted on the top of nozzle block 93 and is securedthereto by a plurality of screws 125.

Horizontal armature 11 is shown sealingly mounted intermediate its ends on the upper end of an upright flexure tube member 126. This member includes a thin walled tubular section capable of flexing or deflecting which surrounds flapper 13 in slightly spaced relation, and also includes an enlarged attaching base sealingly connected to the top of nozzle block 93 by O-ring 128 and fasteners 129. The annular clearance surrounding that portion of flapper 13 arranged within flexure tube member 126 communicates with the upper end of opening 98 in nozzle block 93 and forms part of sump chamber 120. In this manner, flexure tube member 126 supports armature 11 so as to allow pivotal movement thereof while isolating the elements of electrical torque motor 10 arranged externally of the flexure tube from the elements communicatively associated with and wetted by hydraulic fluid on the interior of this tube.

A vertically disposed feedback spring member 131 is cantilever-mountedat its upper end on the lower end of flapper 13 and its lower end has a substantially frictionless connection with valve spool 17 by virtue of a ball enlargement 132 having rolling contact with the walls of an annular groove 133 provided in center lobe 68 of this valve spool.

An electrical connector 134 operatively associated with coils 124 in a well-known manner is provided for electrical signal command input to the servovalve. A motor cap 135 covers torque motor 10 and is sealingly mounted on valve body block 18 by an O-ring 136 and screws 138.

The servovalve as a unit may be mounted on any suitable support (not shown) by fasteners (not shown) extending through vertical mounting holes 139 provided in valve body block 18 adjacent its four corners as shown in FIG. 3. I

The construction and mode of operation of electrohydraulic servovalve described hereinabove is well known to those skilled in the art. Suffice it to say here than an electrical command input signal to the servovalve will produce a proportionate hydraulic response incontrol ports C and C which may be utilized to drive a load. Such input signal energizes coils 124 to induce electromagnetically pivotal movement of armature lland hence flapper 13. Movement of this flapper relative to nozzles 99 and produces differentially variable orifices through which these nozzles discharge fluid. This adjusts the pressures in spool end chambers 91 and 92 to apply a differential pressure against the ends of valve spool 17 and cause the same to displace axially. The direction and extent of such displacement depends upon the polarity and magnitude of the electrical input signal. Displacement of valve spool 17 will establish communication between one of the control ports C and C with either pressure port P or return port R. For a given input signal, the valve spool will displace until feedback spring member 131 bends to create a torque on armature 11 substantially counterbalancing the electrically induced torque thereon. Hence spool displacement will be proportional to signal input.

As explained in the forepart of this specification such an electrohydraulic servovalve is prone to have its slidable valve spool oscillate at one or more characteristic resonant frequencies when the servovalve is operating at elevated pressures and especially with low'viscosity hydraulic fluid. These oscillations produce pressure variations in the spool end chambers 91 and 92. The present invention provides a solution for this problem.

In accordance with the improvement of the present invention, acoustic filter means is provided for each of the spool end chambers 91 and 92 to provide a wave tube having an open end communicating with its companion spool end chamber and having a length to provide a characteristic anti-resonant frequency. Prefera' bly the end of the wave tube remote from the spool end chamber with which it is associated is closed. Also, the wave tube preferably has a length substantially equal to one-quarter the wave length of the characteristic resonant frequency to be attenuated. Further, the volume of the wave tube should be at least one-half of the volume of the spool end chamber with which it is associated. Preferably, such volumes are substantially equal to each other.

Adverting to the drawings, the wave tube designated 91 is associated with left spool end chamber 91 and the wave tube designated 92' is associated with right spool end chamber 92. Such wave tubes 91 and 92 are preferably achieved by drillings provided in the body of the servovalve. Thus, intermediate body block 18 is shown as provided with a pair of parallel horizontally spaced holes 140 and 141 extending horizontally from left end 22 to right end face 23. These holes are arranged at a level above the bushing bore 21 on opposite sides of the centrally disposed horizontal sections of passages 103 and 110 which are aligned.

Left end cap 19 has an upwardly and outwardly inclined drilled recess 142 which at its lower end communicates with annular cap recess 26, as shown in FIG. 6. The upper end of this recess intercepts the inner end of a horizontal recess 143 drilled from cap end face 24 and communicating with the left end of hole 140 in block end face 22. Right end cap 20 has a horizontal dead-end recess 144 drilled from cap end face 31 and which is aligned with and communicates with the right end of hole 140 in block end face 23. An O-ring 145 seals the interface joint between the ends of recess 143 and hole 140, and an O-ring'146 seals the interface joint between the ends of recess 144 and hole 140.

Similarly for the other spool end chamber, right end cap 20 has an upwardly and outwardly inclined drilled recess 148 which at its lower end communicates with annular cap recess 33, as shown in FIG. 7. The upper end of this recess intercepts the inner end of a horizontal recess 149 drilled from cap end face 31 and communicating with the right end of hole 141 in block end face 23. Left end cap 19 has a horizontal dead-end recess 150 drilled from cap end face 24 and which is aligned with and communicates with the left end of hole 141 in block end face 22. An O-ring 151 seals the interface joint between the ends of recess 149 and hole 141, and an O-ring 152 seals the interface joint be tween the ends of recess 150 and hole 141.

Thus, the end-connected drillings 143, 140 and 144 provide wave tube 91 for left spool end chamber 91 of the requisite length and diameter, open at one end and communicating with this chamber via inclined passage 142 and closed at its other end remote from this chamber. Likewise, the end-connected drillings 149, 141 and 150 provide wave tube 92 for right spool end chamber 92 of the requisite length and-diameter, open at one end and communicating with this chamber via inclined passage 148 and closed at its other end remote from this chamber.

An electrohydraulic servovalve of the construction illustrated and described herein, before being provided with wave tubes 91 and 92 tended to oscillate at about 3.3 KHz when operating at a high supply pressure in the range of from 3,000 to 4,000 psi (210 to 280 bars) and the problem was particularly severe with a low viscosity hydraulic fluid as, for example, with a fluid identified as MIL-H5606 at 140F. (60C.). The same type servovalve, when provided with the wave tubes 91 and 92' of the present invention filtered out the 3.3 KI-lz pressure variations in the spool end chambers. Any steady-state variations in end chamber pressure are cancelled by a pressure wave that travels the one-half wavelength path to the blind or closed end of the tube and back, and returns 180 out-of-phase with the input pressure wave. In essence, the 3.3 KHz pressure variations are moved to the blind end of the tubes where they cannot create forces on the valve spool, and the pressure variations at the input to the tubes at their open ends which communicate with their companion spool end chambers are effectively cancelled or at least minimized. In this manner, the oscillation is suppressed.

For 3.3 KI-lz, the required tube length (one-fourth wavelength) is:

M4 v/4f= 1050/(4) (3300) 0080 meter 8.0 centimeters where wave length of the frequency in terms of meters v velocity of sound in the medium in terms of meters per second f= frequency in Hz (Hertz) The 1,050 m/sec velocity of propagation of sound in oil is based on a bulk modulus of 10,000 kg/cm and a fluid density of 0.88.

In the actual exemplary valve mentioned, the actual length of each wave tube 91', 92 was about 8.0 cm, and for tube 91 was composed of the sum of 6.5 cm for the length of hole 140, plus'0.5 cm for the length of blind end recess 144 and 1.0 cm for the length of recess 143; and for tube 92' was composed of the sum of 6.5 cm for the length of hole 141, plus 0.5 cm for the length of blind end recess and 1.0 cm for the length of recess 149. The diameter of each of the various passages 140, 141, 143, 144, 149, 150 was 3.2 mm.

The provision of the acoustic filter means in the form of these wave tubes 91' and 92 provides antiresonance of a characteristic frequency by creating a pressure node at each spool end chamber.

The effects of flow and pressure in a wave tube is graphically illustrated in FIG. 9. At the base of this figure, there is schematically represented the spool end chamber connected to the open end of a closed end wave tube having a length equalto one-quarter the wavelength, )1 of the characteristic resonant frequency of the servovalve to be attenuated. Intermediate is a graph representing the envelope of the resonant pres sure peaks, P(x), as a function of distance, x along the length of the tube according to the following equation;

P(x) P sin (2 'n' x/lt wherein Q is the peak flow (i.e., the'maximum amplitude of the resonant flow peaks).

It will be seen that the zero node of the flow curve, Q(x), is at the right or closed end of the wave tube, and

the antinode or peak flow, Q is at the left or open end of the wave tube.

The foregoing has described the addition of a single pair of anti-resonant wave tubes to the spool end chambers of a servovalve to suppress a single characteristic frequency. It is known to those skilled in the art that some servovalves exhibit several characteristic resonant frequencies such as a first-stage resonant frequency, a second-stage resonant frequency, and others. With servovalves that exhibit multiple characteristic resonant frequencies it may be desirable to provide several acoustic anti-resonant filter means, each of such means tuned to a specific resonant frequency.

It would further be understood by one skilled in the art that certain variations of the acoustic wave tube filter will produce similar anti-resonance for a characteristic frequency. The foregoing embodiment of the present invention utilizes a closedend wave tube having a length equal to one-quarter wavelength (A A) of the characteristic resonant frequency. It is known that other closed-end wave tubes having lengths corresponding to odd integer multiples of the one-quarter wavelength will produce essentiallyidentical antiresonance for the same characteristic resonant frequency. Thus, closed-end wave tubes of lengths A A, A, /4 )t, etc. could equally well be utilized. Such variations may be better suited to the size and detail design of a specific servovalve.

Another variation of acoustic wave tube that will produce similar anti-resonance for a characteristic frequency is an openended wave tube having a length equal to one-half wavelength )t). Likewise, other open-ended wave tubes having integer multiples of one-half wavelength will produce essentially indentical anti-resonance for the same characteristic resonant frequency. Thus open-ended wave tubes of lengths A, )t, 3/2 )t, etc. could equally well be utilized.

From the foregoing it will be seen that the preferred embodiment of the present invention illustrated and described accomplishes the stated objects. Since variations may occur to those skilled in the art without departing from the spirit of the present invention, the scope of the present invention is to be determined by the appended claims.

What is claimed is:

1. In an electrohydraulic servovalve including a body having a compartment in which a valve spool is slidably arranged leaving chambers at opposite ends of said spool, said servovalve being prone to oscillate at one or more characteristic resonant frequencies resulting in pressure variations in such spool end chambers, the improvement thereof which comprises acoustic filter means for each of said spool end chambers and severally arranged to provide a wave tube having an open end communicating with its corresponding one of said spool end chambers and having a length to provide anti-resonance for one characteristic frequency.

2. An electrohydraulic servovalve according to claim 1 wherein the end of each of said wave tubes remote from its corresponding one of said spool end chambers is closed.

3. An electrohydraulic servovalve according to claim 2 wherein said length equals substantially an odd integer multiple of one-quarter wavelength of said characteristic resonant frequency to be attenuated.

4. An electrohydraulic servovalve according to claim 3 wherein the volume of each of said wave tubes is substantially equal to the volume of its corresponding one of said spool end chambers.

5. An electrohydraulic servovalve according to claim 3 wherein the volume of each of said wave tubes is at least one-half of the volume of its corresponding one of said spool end chambers.

6. An electrohydraulic servovalve according to claim 5 wherein said wave tubes are provided by drillings in said body.

7. An electrohydraulic servovalve according to claim 6 wherein said body comprises a block and first and second end caps at opposite ends of said block, said first and second end caps severally in part defining first and second spool end chambers, one of said wave tubes includes a first intermediate drilling extending through said block, a first connecting drilling in such first end cap connecting one end of said first intermediate drilling with said first spool end chamber and a first deadended drilling in said second end cap communicating with the other end of said first intermediate drilling, and the other of said wave tubes includes a second intermediate drilling extending through said block, a second connecting drilling in said second end cap connecting one end of said second intermediate drilling with said second spool end chamber and a second deadended drilling in said first end cap communicating with the other end of said second intermediate drilling.

UNETED A'EES RATENT OFFICE QERTIFKCATE F CWEUNON 3'857'54l December 31, 1974 1 Patent No. Dated Inventor(a) Daniel C- Clark It is certifiefi that error appears in the awe-identifiedpatent and that said Letters Patentare herehy untreated as show below Col. 1, line 34.: "selfoscillation"should be-self oscillation--; Col. 4, line 64: "while" should be-While-;-. Col. 6, line-l0: "7" \should be 76--*;

. line 52; "firststage" should be'firststage-; Col. 7, line 36: "sum" should be sump.; v Col. 8, line' 23:, between "of" and '"electroinsertthe-; Col. 9, line 13: between "end" and "22" in'sert.fa1ce--;' Col. 10, line 53: between "peaks" and insert-)'-'--; Col. 11, line 18: "closedend" should be-*-closed end--;

, line 31: "openended" should beope1 -end ed.

Signed and sealed this llth day 0? March 1 975.

(SEAL) Attest: I

C. MARSHALL DANN RUTH C. MASDN Commissioner of Patents Attesting Officer s and Trademarks- 

1. In an electrohydraulic servovalve including a body having a compartment in which a valve spool is slidably arranged leaving chambers at opposite ends of said spool, said servovalve being prone to oscillate at one or more characteristic resonant frequencies resulting in pressure variations in such spool end chambers, the improvement thereof which comprises acoustic filter means for each of said spool end chambers and severally arranged to provide a wave tube having an open end communicating with its corresponding one of said spool end chambers and having a length to provide anti-resonance for one characteristic frequency.
 2. An electrohydraulic servovalve according to claim 1 wherein the end of each of said wave tubes remote from its corresponding one of said spool end chambers is closed.
 3. An electrohydraulic servovalve according to claim 2 wherein said length equals substantially an odd integer multiple of one-quarter wavelength of said characteristic resonant frequency to be attenuated.
 4. An electrohydraulic servovalve according to claim 3 wherein the volume of each of said wave tubes is substantially equal to the volume of its corresponding one of said spool end chambers.
 5. An electrohydraulic servovalve according to claim 3 wherein the volume of each of said wave tubes is at least one-half of the volume of its corresponding one of said spool end chambers.
 6. An electrohydraulic servovalve according to claim 5 wherein said wave tubes are provided by drillings in said body.
 7. An electrohydraulic servovalve according to claim 6 wherein said body comprises a block and first and second end caps at opposite ends of said block, said first and second end caps severally in part defining first and second spool end chambers, one of said wave tubes includes a first intermediate drilling extending through said block, a first connecting drilling in such first end cap connecting one end of said first intermediate drilling with said first spool end chamber and a first dead-ended drilling in said second end cap communicating with the other end of said first intermediate drilling, and the other of said wave tubes includes a second intermediate drilling extending through said block, a second connecting drilling in said second end cap connecting one end of said second intermediate drilling with said second spool end chamber and a second dead-ended drilling in said first end cap communicating with the other end of said second intermediate drilling. 